Fact Sheet 4.10. Agroforests at the Margins of the Humid Tropics

The complex agroforests of Indonesia are indigenous systems invented over generations
by local people living at the margins of tropical rainforests in Sumatra (Torquebiau,
1984). After slash-and-burn of a primary forest, food crops are planted along
with coffee, pepper, fruit trees (Lansium domesticum-duku, Durio zibethinus-durian),
and the resin-producing damar tree (Shorea javanica). The trees eventually
shade out the crops, occupy different strata, and produce high-value products
such as fruits, resins, medicinals and high-grade timber (de Foresta and Michon,
1994). Biophysical scientists have studied the productivity and ecological dimensions
of these systems (Michon and de Foresta, 1996). Villagers in Krui, Lampung Province,
who live off these complex agroforests obviously have a higher standard of living
than those that grow only crops (Bouamrane, 1996). Detailed explanations of
other best-bet agroforestry activities are described elsewhere, including activities
that start from secondary forest fallows, reclamation of abandoned Imperata
grasslands (Garrity, 1997; Friday et al., 1999), or degraded pastures
in the Amazon.

Use and Potential
The greatest potential area for expanding agroforestry practices and other forms
of land-use intensification is in areas considered "degraded" at the margins
of the humid tropics, such as many secondary forest fallows, Imperata grasslands,
and degraded pastures. These areas amount to about 250 Mha, or 42 percent of
the total deforested area of the humid tropics (Sanchez et al., 1994).
A major advantage of such lands is their proximity to roads and urban areas
because they often were the first ones to be cleared. We assume that 3 percent
of these lands (7.5 Mha) plus 20 percent of the 15 Mha annually deforested (3
Mha)-a total of 10.5 Mha-can be put into agroforestry yearly with enabling government
policies such as those described by Fay et al. (1998) and Tomich et
al. (1998).

Methods
Changes in time-averaged aboveground and soil carbon can be measured as described
elsewhere in this Special Report. Newly developed allometric equations for estimating
biomass of shrubs and small trees provide an important tool (Palm et al.,
2000).

Current Knowledge and Scientific Uncertainties
Data by Palm et al. (2000) can be used to calculate the difference between
carbon lost when moist tropical forests are transformed into agroforests and
carbon lost when forests are transformed into croplands or pastures. In this
case, the estimate is 46 t C ha-1. The largest uncertainty is the area that
will benefit from such practices. Other human-induced activities at the forest
margins that do not include trees (e.g., active slash-and-burn cropping phase,
pastures, and grasslands) are sources rather than sinks of carbon.

Monitoring, Verifiability, and Transparency
A 10-year period is recommended to assess the impact on soil carbon stocks.
Direct measurement (time-averaged) of aboveground and soil stocks should be
used for monitoring. Combining present algorithms for estimating biomass in
shrubs and small trees (Palm et al., 2000), standard soil carbon sampling,
and GIS techniques appears feasible. The level of carbon stock change can be
readily verified through land-based techniques described above. Assumptions
and methodologies associated with this practice can be explained clearly to
facilitate replication and assessment. Scientific methods are open to review
and replicable over time.

Permanence
Aboveground carbon stocks can be rapidly eliminated by shifting from this practice
to slash-and-burn, followed by cropland or pasture. About half of the carbon
stored in the soil is likely to have a turnover rate of >20 t C yr-1. Stopping
the activity would lead to an estimated loss of soil carbon of 50 percent in
about 5 years.

Associated Impacts
Agroforestry is an economic activity that helps to reduce or eliminate poverty
at the forest margins if high-value products (fruits, resins, medicines, and
high-grade timber) are produced. Agroforestry also facilitates land tenure because
it encourages settled farming systems (Fay et al., 1998). Tree-based
systems can serve as a methane sink (Murdiyarso et al., 1994; Palm et
al., 2000). Methane emitted by 1 ha of paddy rice can be absorbed by 24
ha of agroforests if the patches are close enough in a landscape mosaic. There
seems to be no difference in N2O emissions between the original forest, agroforests,
cropland, or grassland at the humid tropical forest margins (Palm et al.,
2000).

Agroforestry systems are more biodiverse above ground than crops, grasslands,
and secondary forest fallows in the humid tropics (Gillison, 1999). Differences
in below-ground biodiversity seem less important (Swift, 1999). Plant diversity
in mature complex agroforests of Indonesia is on the order of 300 species per
hectare, which approximates that of adjacent undisturbed forests (420 plant
species per hectare). The diversity of bird species in these agroforests is
approximately half that of the original rainforest, and almost all mammal species
are present in the agroforest (de Foresta and Michon, 1994). This biodiversity
is possible because such agroforests, which are composed of hundreds of small
plots that are managed by individual families, occupy contiguous areas of several
thousand hectares in Sumatra. Such agroforestry "corridors" are an important
tool for avian biodiversity conservation in Central America (Current et al.,
1998).

In view of human migrations to the forest margins, the optimal tradeoffs between
carbon capture and economic and social benefits are an important policy determination.
Examples of such tradeoffs are described by Gokowski et al. (1999), Vosti
et al. (1999), and Tomich et al. (1998, 1999).

Relationship to IPCC Guidelines
Changes in woody biomass stocks associated with agroforestry practices can be
included by using the procedures for estimating "changes in forest and other
woody biomass stocks." The Guidelines do not provide specific default values
for agroforest systems, however. Changes in soil carbon stocks can also be estimated
using the Guidelines, although specific examples and default values for agroforest
systems are not provided. Soil carbon stock change estimates would require values
for "Input Factors," and potentially "Tillage Factors," related to the level
of management of agroforests. For land-use conversions to agroforestry, "Base
Factors" exist in the Workbook for conversions from forest and from cultivated
land.